Hepatic fibrosis is examined in mice infected with schistosome species pathogenic for man. Mouse strains develop markedly different degrees of hepatic fibrosis following infection with S. mansoni. Among inbred strains, Nmri mice show maximal fibrosis and C57BL/6 mice minimal fibrosis. Inheritance was multigenic with incomplete dominance. Different mouse strains showed similar patterns of fibrosis whether infected with S. japonicum or S. mansoni. S. haematobium, which does not produce hepatic fibrosis in man, also induced marked fibrosis in the mouse liver.